A neuron is a main element controlling the life activity of an individual higher organism. It had been thought that neurons of the certain nervous system perform neither postnatal differentiation nor regeneration, but only deciduate from one minute to the next. However in the 1990s, neural stem cells which had not yet differentiated into neurons were first found in a fetal brain, and as a result of further demonstration of the presence of neural stem cells in an adult brain, a possibility of regeneration of the central nervous system has been shown. Thus, a possible therapy for intractable neuronal diseases using tissue stem cells such as neural stem cells, and embryonic stem cells (ES cells) are in the limelight. However, there are problems that somatic stem cell such as neural stem cell as well as embryonic stem cell, even when it is transplanted as it is, does not differentiate into neuronal cell (neuron) and has a difficulty even in taking, and that most of them, even when taken, differentiate into glial cells. In addition, neuronal cells (neuron) which are easiest to induce differentiation into neural stem cells are difficult to collect from the brain of the same individual, and when human fetal brain is used, problems associated with ethical issues and rejection reaction must be overcome. In the case of bone marrow interstitial cells, skin stem cells and adipose stem cells, which are reported to differentiate into other neuronal cells, they are difficult to induce neuronal differentiation, and thus a technique for induction to neuron at a high yield for a short period of time has not yet been established.
Regarding these problems, we have considered that VHL gene and VHL protein may play a role from the developmental stage of neurons based on the fact that they are specifically expressed in neurons of the central nervous system, and then we have studied the expression of VHL protein in neural stem cells. Since VHL protein is expressed mainly in the cytoplasm as neural stem cells differentiate into neurons, and further, introduction of VHL gene into neural stem cells using a viral vector promotes differentiation into neurons while neuronal stem sell as it is inhibits inversely differentiation into neuron by inhibiting the action of VHL gene using antisense oligonucleotide which is an inverse sequence of messenger ribonucleic acid (RNA) of VHL gene, it has now been clarified that VHL gene has an ability to induce neuronal differentiation (non patent literature 1). It is affirmed that this ability of VHL gene to induce neuronal differentiation is a common phenomenon in neuroblastoma (non patent literature 2), ES cell, skin stem cell and bone marrow stromal cell. Nurr 1 and Mash 1 are known to be a gene showing an ability to induce neuronal differentiation like VHL gene, but only a method of gene transfer into a cell by using some vector has been reported until now. Thus, any trial to induce neuronal differentiation by introducing into a stem cell a protein having a powerful ability to induce neuronal differentiation instead of the gene has not yet been made. Furthermore, a method of activating an endogeneous neural stem cell and promoting regeneration of nerve by introducing into a body a protein having an ability to induce neural differentiation has not yet been developed. The preparation of a chemically synthesizable oligopeptides showing an ability to induce neuronal differentiation has not been reported. In addition, the utilization of such oligopeptides for regeneration of nerve has also not been reported.
In the United States, clinical trials using fetal brain obtained by artificial termination of pregnancy for treating Parkinson's disease have been already conducted and a certain effect has been recognized. At the level of animal experiment, a trial, in which neural stem cells or ES cells are grafted to the brain or spinal cord, and the cells are allowed to differentiate into neurons. has been started to treat intractable neuronal diseases including not only Parkinson's disease but also brain infarction, spinal cord injury and the like. Moreover, regeneration of peripheral nerve in vitro or in vivo in the form of bundles of nerve fibers has already been attempted. However, since neurons do not principally divide and proliferate, it is difficult to form practical bundles of nerve. In nerve grafting to treat ruptured peripheral nerve, normally the autologous nerve of a lower limb is excised and grafted. However, production of artificial nerve for nerve grafting in place of autologous nerve has not been successful.    [non patent literature 1] Kanno H et al.: Cancer Res 60: 2820–4, 2000    [non patent literature 2] Murata H, et al.: Cancer Res 62: 7004–7011, 2002